The present invention relates to a light guiding plate for guiding light from a light source to illuminate a liquid crystal cell.
Generally, a liquid crystal display has a light guiding plate to guide beams of light from a light source and radiate the light onto the entire surface of the cell. The light guiding plate is, for example, located on the observer""s side of a reflective liquid crystal cell and functions as a front light, which radiate light onto the observer""s side.
FIGS. 14(A) and 14(B) show a prior art light guiding plate 9, and FIGS. 15(A) and 15(B) show a prior art light guiding plate 90. A light source 2 is located at a side of each light guiding plate 9, 90. Beams 4 of light emitted from the light source 2 are introduced into the guiding plates 9, 90. When specific conditions are satisfied, the beams 4 are transmitted by a first surface 93 of the plate 9, 90. After being transmitted by the first surface 93, the beams 4 are radiated onto a liquid crystal cell 3 through a flat second surface 92, which faces the cell 3.
As shown in FIG. 14(A), the first surface 93 of the plate 9 has a saw-tooth cross section and grooves 95. Each groove 95 is defined by an inclined face 951 of a first group and an inclined face 952 of a second group. Each first inclined face 951 reflects the beams 4 from the light source 2 and guides the beams 4 toward a side 99 of the plate 9 that is located at opposite side to the light source 2. Each second inclined face 952 reflects the beams 4 from the light source 2 and toward the second surface 92 so that the second surface 92 emits the beams 4.
The beams 4 in the guiding plate 9 propagate through the guiding plate 9 while being reflected at the second surface 92 and the first inclined faces 95. During the propagation, the beams 4 are reflected by the second inclined faces 952 and are transmitted by the second surface 92. Then, the beams 4 are radiated onto the cell 3.
The beam 4 is totally guided by one of the first inclined faces 951 along a direction away from the light source 2 with a changed propagation angle. Therefore, the beam 4 is also radiated onto the cell 3 at a location that is relatively far from the light source 2.
The propagation angle refers to the angle defined by the second surface 92 and the vector of a propagating beam.
In the light guiding plate 90, which is shown in FIGS. 15(A) and 15(B), V-shaped grooves 96 are formed on the first surface 93. Also, a reflector 91 is located at the side that is opposite to the light source 2.
Each groove 96 has an inclined face 962 of a first group and an inclined face 961 of a second group. Each first inclined face 962 reflects the beams 4 from the light source 2 and to the second surface 92 so that the second surface 92 emits the beams 4. Each second inclined face 961 reflects beams 41 of light that have been reflected by the reflector 91 to the second surface 92 so that the second surface 92 emits the beams 4.
Each part of the first surface 93 where there is no groove 96 is flat and is referred to a flat face 963.
The beams 4 in the guiding plate 90 propagate through the guiding plate 90 while being reflected by the second surface 92 and the flat faces 963. During the propagation, the beams 4 are reflected by the first inclined faces 962 or the second inclined faces 961 and are transmitted by the second surface 92. Then, the beams 4 are radiated onto the cell 3.
The light guiding plate 9 shown in FIGS. 14(A) and 14(B) does not have a reflector at the side 99 opposite to the light source 2. Therefore, some of the light that reaches the side 99 passes through the side 99. As a result, the light that has passed through the side 99 cannot be radiated onto the cell 3. Therefore, the intensity of light from the light guiding plate 9 is insufficient for the flat panel light source.
The plate 9 has no reflector because, unlike the plate 90, no surfaces that reflect beams from the side 99 and guide the beams to the cell 3 are formed on the first surface 93. That is, even if there were a reflector, beams reflected by the reflector would not be guided to be transmitted by the second surface 92.
The guiding plate 90 of the FIGS. 15(A) and 15(B) has the reflector 91 and the second inclined faces 961. The second inclined faces 961 reflect the beams 41, which have been reflected by the reflector 91, to the second surface 92. Therefore relatively less light is wasted.
However, in the area that is relatively far from the light source 2, the intensity of light that passes through the second surface 92 is relatively weak, which causes uneven brightness.
The reason for the uneven brightness is considered to be as follows.
The beams 4 that enter the plate 90 have substantially even propagation angles. Also, after being reflected by the first and second inclined faces 962, 961, the beams 4, 41 are transmitted by the second surface 92 only when the propagation angles of the beams 4, 41 satisfy a limited condition of propagation angle. Therefore, more beams 4 which propagation angles satisfy the condition exit the plate 90 at an area relatively close to the light source 2. This reduces the amount of beams 4 that reach areas far from the light source 2. Accordingly, the brightness is uneven.
Accordingly, it is an objective of the present invention to provide a light guiding plate that has a sufficient and even brightness.
To achieve the above objective, the present invention provides a light guiding plate for guiding light from a light source to illuminate a liquid crystal cell. The light guiding plate comprises a reflector located on an opposite side of the plate from the light source. The reflector reflects light from the light source. A first surface selectively transmits or reflects light. A second surface, which is flat, is located between the cell and the first surface. The second surface selectively transmits or reflects light. A first region is formed on the first surface at a location that is relatively near the light source. The first region includes a first inclined face and a second inclined face. The first inclined face reflects light from light source to guide light to the reflector. The second inclined face reflects light from the light source to the second surface so that the second surface emits light. A second region is formed on the first surface at a location that is relatively far from the light source. The second region includes a third inclined face and a fourth inclined face. The third inclined face reflects light from the light source to the second surface so that the second surface emits light. The fourth inclined face reflects light reflected by the reflector to the second surface so that the second surface emits light.
The present invention also provides a light guiding plate for guiding light from a light source to illuminate a liquid crystal cell. The light guiding plate comprises a reflector located on an opposite side of the plate from the light source. The reflector reflects light from the light source. A first surface selectively transmits or reflects light. A second surface, which is flat, is located between the cell and the first surface. The second surface selectively transmits or reflects light. A plurality of recesses are located on the first surface at predetermined intervals. Each recess includes a first inclined face and a second inclined face. The first inclined face reflects light from the light source to the second surface so that the second surface emits light. The second inclined face reflects light reflected by the reflector to the second surface so that the second surface emits light. A plurality of third inclined faces are located on the first surface. Each third inclined face is located between a pair of adjacent recesses and is inclined relative to the second surface.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.